The present invention relates to a method and an apparatus for changing a printing plate.
In sheet-fed offset printing machines, the printing formes, designed as printing plates, are held on the plate cylinder (forme cylinder) by a respective fixing device, configured as a clamping rail, at the location corresponding to the start of the print and the location corresponding to the end of the print. After the printing plate has been fixed in position, the plate is tensioned through the exertion of a tensile force.
To remove a used printing plate from the plate cylinder, the fixing device assigned to the end of the print is first loosened, so as to free the end of the printing plate. By rotating the plate cylinder backwards (i.e. the direction opposite to the direction of rotation during printing), the printing plate is led off the plate cylinder. This is accomplished by rotating the plate cylinder backwards until it has assumed a position in which, after the fixing device assigned to the start of the print has been loosened, the printing plate can be removed completely from the cylinder. Semi-automatic or fully automatic printing-plate changers can be used, by means of which the printing plate can be removed from the cylinder and from the printing unit and/or transferred into a storage chamber or another holding device.
To feed a new printing plate, the cylinder is first moved into a position in which a printing plate brought up tangentially with respect to the cylinder can be inserted with the edge assigned to the start of the print into a corresponding fixing device. After the appropriate edge of the printing plate has been inserted into the fixing device assigned to the start of the print, the printing plate is fixed (clamped) to the plate cylinder. The printing plate cylinder is then rotated in the forward direction until the printing plate has been drawn nearly completely around the outer circumference of the plate cylinder. As the plate cylinder is rotated, an appropriate tensile force is exerted on the printing plate by means of additional pressure rollers, such that the printing plate is placed smoothly onto the outer circumference of the cylinder. The forward rotation of the plate cylinder continues until the edge of the printing plate corresponding to the print end can be inserted by means of an associated pressure roller into the fixing device on the cylinder assigned to the print end.
After the trailing edge of the printing plate (i.e. the end of the plate corresponding to the print end) has been fixed (clamped) to the plate cylinder, the printing plate is tensioned by building up a tensile force on the print-end fixing device. In particular, this tensile force can be applied by spring elements. Devices for the automated changing of printing plates on a plate cylinder, in which the operations described above are carried out, are disclosed in DE 4215969 C2 and EP 0654349 A1. In addition, EP 0660777 B1 discloses an apparatus for feeding printing plates to the plate cylinder and removing them from the plate cylinder, in which both the feeding and removing operations are carried out with the plate cylinder rotating in a single direction. In particular, the printing plate being removed is released at the start of the print and is conveyed with the print-start edge of the plate at the front into an appropriate storage chamber.
A common factor with all of these devices is that the printing plate is changed from the deliverer side, that is to say from the side of the printing unit which is assigned to the deliverer of the sheet-fed printing machine. In the case of sheet-fed printing machines and, in particular, sheet-fed offset printing machines having an in-line design, the side of the machine assigned to the deliverer is generally easily accessible, i.e. portions of the blanket cylinder and printing plate cylinder are accessible after a slide or folding guard has been opened. The portions of the printing units which face the feeder of the sheet-fed printing machine are primarily occupied by the rolls of the inking and damping units, so that the printing plates cannot be changed from the feeder side of the machine. Accordingly, devices for the semi-automatic and fully automatic changing of printing plates are also fitted on the deliverer side of the printing units.
In sheet-fed printing machines and, in particular, sheet-fed offset printing machines having a verso printing unit upstream of the standard printing units, problems relating to the accessibility of the plate cylinder arise because of the arrangement of the printing plate cylinder in conjunction with its direction of rotation, as well as the interaction of the rolls of the damping and/or inking unit with the plate cylinder. These problems are particularly acute when the verso printing unit is arranged with the appropriate plate cylinder underneath the delivery table of a feeder. A sheet-fed offset printing machine with an upstream verso printing unit is disclosed, for example, by DE 4343616 A1.
Because of the restricted accessibility to the plate cylinder within a verso printing device, the known methods and apparatus for changing a printing plate cannot be used or cannot be used without substantial additional production costs.
Accordingly, in view of the foregoing, a general object of the present invention is to provide a method and a corresponding apparatus by which the printing plate of a plate cylinder of a sheet-fed printing machine, more particularly a plate cylinder of a verso printing device, can be changed simply and without the need for any additional components.
The present invention provides this and other advantages and overcomes the drawbacks of the prior art by providing a method and corresponding apparatus by which the feeding and removal of a printing plate to and from a plate cylinder are carried out using rotational directions of the cylinder which are reversed with respect to conventional methods. In particular, when a printing plate is fed to the plate cylinder, the edge of the plate assigned to the end of the print (i.e., the print-end edge) is first fixed to the plate cylinder fixing device which corresponds to the print end. The plate cylinder is then rotated in the direction opposite to the direction of rotation used during a printing operation, until the new printing plate has been drawn nearly completely around the outer circumference of the plate cylinder. The edge of the printing plate assigned to the start of the print (i.e. the print-start edge) is then fixed to the fixing device corresponding to the print start.
To convey a used printing plate away from the plate cylinder, the print-start edge of the printing plate is first detached (released) from its corresponding fixing device and the plate cylinder is rotated in the direction that is used during a printing operation (i.e. the printing direction). Once the plate cylinder has been rotated in this direction, the printing plate can be removed, for example by hand. Alternatively, an appropriate printing plate holding device can be assigned to the plate cylinder into which the printing plate being removed can be conveyed.
The plate cylinder is rotated in the printing direction until the print-end edge of the printing plate can be released from its corresponding fixing device. Depending on the design of the corresponding fixing device for the print-end edge of the printing plate, the plate cylinder is stopped briefly in this position in order to activate an actuating device associated with the fixing device.
Because a new printing plate is fed to the plate cylinder beginning with the print-end edge of the plate and the plate cylinder is rotated in the direction opposite to the direction used during printing in order to draw the printing plate onto the cylinder, the printing plate can be changed in the area between the blanket cylinder and damping and/or inking unit. This corresponds to the feeder side of both a sheet-fed offset printing machine having an in-line design and a verso printing unit of a sheet-fed offset printing machine having a plate cylinder arranged underneath the corresponding impression cylinder. Thus, the present invention allows the printing plates to be changed from the feeder side in such printing machines.
The method according to the invention is preferably used in plate cylinders of verso printing units of sheet-fed offset printing machines. In the case of sheet-fed offset printing machines having an in-line design, in which there is a verso printing unit arranged upstream from the first printing unit for the recto print, the method of the present invention allows the printing plates in the first verso printing unit can be changed in a simple, straightforward manner. If appropriately designed protective devices are provided for the plate cylinder of the verso printing unit, a printing plate changing operation can be executed in a convenient manner and in which only a few handling operations are required. Likewise, the method and the apparatus of the present invention can be used in instances where the plate cylinders interact directly with an impression cylinder in varnishing units of sheet-fed offset printing machines.
The method of the present invention also permits a plate cylinder to be used in the verso printing unit or in the varnishing unit that is essentially identical in construction and with regard to the arrangement of the fixing devices for the edges of the printing plate as compared to the cylinders of the other printing units. In such a case, the plate cylinder in the verso printing unit rotates in the opposite direction during printing. In other words, the fixing device assigned to the start of the print in the case of the plate cylinder in the recto printing unit is assigned to the end of the print in the plate cylinder of the verso printing unit. Similarly, the fixing device assigned to the end of the print in the recto printing unit is assigned to the start of the print in the plate cylinder of the verso printing unit.
These and other features and advantages of the invention will be more readily apparent upon reading the following description of a preferred exemplary embodiment of the invention and upon reference to the accompanying drawings wherein: